Planetary gear train of automatic transmission for vehicle

ABSTRACT

A planetary gear train of an automatic transmission may include: an input shaft; an output shaft parallel with the input shaft; a first planetary gear set on the input shaft including a first, second and third rotation elements, the first selectively connected to the input shaft, and the third selectively connected to the input shaft; a second planetary gear set on the output shaft including a fourth, fifth and sixth rotation elements, the first selectively connected to the input shaft through two gear ratio paths and selectively connected to a transmission housing, the fifth selectively connected to the input shaft and the transmission housing, and the sixth connected to the second rotation element and directly connected to the output shaft; four transfer gears; and frictional elements including clutches and brakes.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority of Korean Patent ApplicationNumber 10-2012-0124117 filed Nov. 5, 2012, the entire contents of whichapplication is incorporated herein for all purposes by this reference.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to an automatic transmission for avehicle. More particularly, the present invention relates to a planetarygear train of an automatic transmission for a vehicle that can improvemountability and power delivery performance and reduce fuel consumption.

2. Description of Related Art

Typically, a planetary gear train is realized by combining a pluralityof planetary gear sets and friction members. It is well known that whena planetary gear train realizes a greater number of shift speeds, speedratios of the planetary gear train can be more optimally designed, andtherefore a vehicle can have economical fuel mileage and betterperformance. For that reason, the planetary gear train that is able torealize more shift speeds is under continuous investigation.

Though achieving the same number of speeds, the planetary gear train hasa different operating mechanism according to a connection betweenrotation elements (i.e., sun gear, planet carrier, and ring gear). Inaddition, the planetary gear train has different features such adurability, power delivery efficiency, and size depend on the layoutthereof. Therefore, designs for a combining structure of a gear trainare also under continuous investigation.

If the number of shift-speeds, however, increases, the number ofcomponents in the automatic transmission also increases. Therefore,mountability, cost, weight and power delivery efficiency may bedeteriorated.

Particularly, since the planetary gear train having a number ofcomponents is hard to be mounted in a front wheel drive vehicle,researches for minimizing the number of components have been developed.

The information disclosed in this Background section is only forenhancement of understanding of the general background of the inventionand should not be taken as an acknowledgement or any form of suggestionthat this information forms the prior art already known to a personskilled in the art.

BRIEF SUMMARY

Various aspects of the present invention provide for a planetary geartrain of an automatic transmission for a vehicle having advantages ofimproving mountability by shortening a length thereof and reducing thenumber of components as a consequence of achieving eight forward speedsand one reverse speed by disposing two planetary gear sets separately onan input shaft and an output shaft disposed in parallel and connectingrotation elements of the planetary gear sets through a plurality ofexternally-meshed gears.

In addition, the present invention has been made in an effort to providea planetary gear train of an automatic transmission for a vehicle havingfurther advantages of enabling of setting optimum gear ratios due toease of changing gear ratios by using a plurality of externally-meshedgears, and accordingly improving power delivery performance and fueleconomy.

A planetary gear train of an automatic transmission for a vehicleaccording to one aspect of the present invention may include: an inputshaft receiving torque of an engine; an output shaft disposed inparallel with the input shaft with a predetermined distance andoutputting changed torque; a first planetary gear set disposed on theinput shaft, and including a first rotation element selectivelyconnected to the input shaft, a second rotation element operated as anoutput element, and a third rotation element selectively connected tothe input shaft; a second planetary gear set disposed on the outputshaft, and including a fourth rotation element selectively connected tothe input shaft through two paths having different gear ratios andselectively connected to a transmission housing, a fifth rotationelement selectively connected to the input shaft and the transmissionhousing, and a sixth rotation element connected to the second rotationelement and directly connected to the output shaft; four transfer gearsexternally meshing the fourth, fifth, and sixth rotation elements withthe input shaft, the second rotation element or the third rotationelement; and frictional elements including clutches selectivelyconnecting the first, third, and fourth rotation elements with the inputshaft and brakes selectively connecting the fourth and fifth rotationelements with the transmission housing.

The first planetary gear set may be a single pinion planetary gear sethaving a first sun gear being the first rotation element, a first planetcarrier being the second rotation element, and a first ring gear beingthe third rotation element, and the second planetary gear set may be asingle pinion planetary gear set having a second sun gear being thefourth rotation element, a second planet carrier being the fifthrotation element, and a second ring gear being the sixth rotationelement.

The four transfer gears may include: a first transfer gear including afirst transfer drive gear connected to the second rotation element and afirst transfer driven gear connected to the sixth rotation element; asecond transfer gear including a second transfer drive gear connected tothe third rotation element and a second transfer driven gear connectedto the fifth rotation element; a third transfer gear including a thirdtransfer drive gear connected to the input shaft and a third transferdriven gear connected to the fourth rotation element; and a fourthtransfer gear including a fourth transfer drive gear connected to theinput shaft and a fourth transfer driven gear connected to the fourthrotation element.

The frictional elements may include: a first clutch disposed between theinput shaft and the first rotation element; a second clutch disposedbetween the input shaft and the third transfer gear; a third clutchdisposed between the fourth transfer gear and the fourth rotationelement; a fourth clutch disposed between the input shaft and the thirdrotation element; a first brake disposed between the fifth rotationelement and the transmission housing; and a second brake disposedbetween the fourth rotation element and the transmission housing.

The first clutch and the first brake may be operated at a first forwardspeed, the first clutch and the second brake may be operated at a secondforward speed, the first clutch and the second clutch may be operated ata third forward speed, the first clutch and the third clutch may beoperated at a fourth forward speed, the first clutch and the fourthclutch may be operated at a fifth forward speed, the third clutch andthe fourth clutch may be operated at a sixth forward speed, the secondclutch and the fourth clutch may be operated at a seventh forward speed,the fourth clutch and the second brake may be operated at an eighthforward speed, and the third clutch and the first brake may be operatedat a reverse speed.

The frictional elements may include: a first clutch disposed between theinput shaft and the first rotation element; a second clutch disposedbetween the third transfer gear and the fourth rotation element; a thirdclutch disposed between the fourth transfer gear and the fourth rotationelement; a fourth clutch disposed between the input shaft and the thirdrotation element; a first brake disposed between the fifth rotationelement and the transmission housing; and a second brake disposedbetween the fourth rotation element and the transmission housing.

The frictional elements may include: a first clutch disposed between theinput shaft and the first rotation element; a second clutch disposedbetween the input shaft and the third transfer gear; a third clutchdisposed between the fourth transfer gear and the fourth rotationelement; a fourth clutch disposed between the input shaft and the thirdrotation element; a first brake disposed between the third rotationelement and the transmission housing; and a second brake disposedbetween the fourth rotation element and the transmission housing.

The first planetary gear set may be a double pinion planetary gear setincluding a first sun gear being the first rotation element, a firstring gear being the second rotation element, and a first planet carrierbeing the third rotation element, and the second planetary gear set maybe a single pinion planetary gear set including a second sun gear beingthe fourth rotation element, a second planet carrier being the fifthrotation element, and a second ring gear being the sixth rotationelement.

A planetary gear train of an automatic transmission for a vehicleaccording to another aspect of the present invention may include: aninput shaft receiving torque of an engine; an output shaft disposed inparallel with the input shaft with a predetermined distance andoutputting changed torque; a first planetary gear set disposed on theinput shaft, and including a first rotation element selectivelyconnected to the input shaft, a second rotation element, and a thirdrotation element selectively connected to the input shaft; a secondplanetary gear set disposed on the output shaft, and including a fourthrotation element selectively connected to the input shaft through twopaths having different gear ratios and selectively connected to atransmission housing, a fifth rotation element selectively connected tothe input shaft and the transmission housing, and a sixth rotationelement connected to the second rotation element and directly connectedto the output shaft; a first transfer gear connecting the secondrotation element with the sixth rotation element; a second transfer gearconnecting the third rotation element with the fifth rotation element; athird transfer gear connecting the input shaft with the fourth rotationelement; a fourth transfer gear connecting the input shaft with thefourth rotation element; and frictional elements including clutchesselectively connecting the first, third, and fourth rotation elementswith the input shaft and brakes selectively connecting the fourth andfifth rotation elements with the transmission housing.

The first planetary gear set may be a single pinion planetary gear sethaving a first sun gear being the first rotation element, a first planetcarrier being the second rotation element, and a first ring gear beingthe third rotation element, and the second planetary gear set may be asingle pinion planetary gear set having a second sun gear being thefourth rotation element, a second planet carrier being the fifthrotation element, and a second ring gear being the sixth rotationelement.

The first planetary gear set may be a double pinion planetary gear setincluding a first sun gear being the first rotation element, a firstring gear being the second rotation element, and a first planet carrierbeing the third rotation element, and the second planetary gear set maybe a single pinion planetary gear set including a second sun gear beingthe fourth rotation element, a second planet carrier being the fifthrotation element, and a second ring gear being the sixth rotationelement.

The frictional elements may include: a first clutch disposed between theinput shaft and the first rotation element; a second clutch disposedbetween the input shaft and the third transfer gear; a third clutchdisposed between the fourth transfer gear and the fourth rotationelement; a fourth clutch disposed between the input shaft and the thirdrotation element; a first brake disposed between the fifth rotationelement and the transmission housing; and a second brake disposedbetween the fourth rotation element and the transmission housing.

The frictional elements may include: a first clutch disposed between theinput shaft and the first rotation element; a second clutch disposedbetween the third transfer gear and the fourth rotation element; a thirdclutch disposed between the fourth transfer gear and the fourth rotationelement; a fourth clutch disposed between the input shaft and the thirdrotation element; a first brake disposed between the fifth rotationelement and the transmission housing; and a second brake disposedbetween the fourth rotation element and the transmission housing.

The frictional elements may include: a first clutch disposed between theinput shaft and the first rotation element; a second clutch disposedbetween the input shaft and the third transfer gear; a third clutchdisposed between the fourth transfer gear and the fourth rotationelement; a fourth clutch disposed between the input shaft and the thirdrotation element; a first brake disposed between the third rotationelement and the transmission housing; and a second brake disposedbetween the fourth rotation element and the transmission housing.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary planetary gear trainaccording to the present invention.

FIG. 2 is an operational chart of friction elements at each shift-speedapplied to the planetary gear train of FIG. 1.

FIG. 3A is a lever diagram of the planetary gear train of FIG. 1 at thefirst forward speed and the second forward speed.

FIG. 3B is a lever diagram of the planetary gear train of FIG. 1 at thethird forward speed.

FIG. 3C is a lever diagram of the planetary gear train of FIG. 1 at thefourth forward speed.

FIG. 3D is a lever diagram of the planetary gear train of FIG. 1 at thefifth forward speed.

FIG. 3E is a lever diagram of the planetary gear train of FIG. 1 at thesixth forward speed.

FIG. 3F is a lever diagram of the planetary gear train of FIG. 1 at theseventh forward speed.

FIG. 3G is a lever diagram of the planetary gear train of FIG. 1 at theeighth forward speed.

FIG. 3H is a lever diagram of the planetary gear train of FIG. 1 at thereverse speed.

FIG. 4 is a schematic diagram of an exemplary planetary gear trainaccording to the present invention.

FIG. 5 is a schematic diagram of an exemplary planetary gear trainaccording to the present invention.

FIG. 6 is a schematic diagram of an exemplary planetary gear trainaccording to the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

Description of components that are not necessary for explaining variousaspects of the present invention will be omitted, and the sameconstituent elements are denoted by the same reference numerals in thisspecification.

In the detailed description, ordinal numbers are used for distinguishingconstituent elements having the same terms, and have no specificmeanings.

FIG. 1 is a schematic diagram of a planetary gear train according tovarious embodiments of the present invention.

Referring to FIG. 1, a planetary gear train according to variousembodiments of the present invention includes a first planetary gear setPG1 disposed on an input shaft IS, a second planetary gear set PG2disposed on an output shaft OS disposed in parallel with the input shaftIS, and frictional elements consisting of four clutches C1, C2, C3, andC4 and two brakes B1 and B2.

Therefore, torque input from the input shaft IS is converted into eightforward speeds and one reverse speed by cooperation of the first andsecond planetary gear sets PG1 and PG2, and then is output through theoutput shaft OS.

The input shaft IS is an input member, and torque from a crankshaft ofthe engine is changed through a torque converter and is then input tothe input shaft IS.

The output shaft OS is an output member and transmits driving torque toa driving wheel through a differential apparatus.

The first planetary gear set PG1 is a single pinion planetary gear set,and includes a first sun gear S1 being a first rotation element N1, afirst planet carrier PC1 rotatably supporting a first pinion P1externally meshed with the first sun gear Si and being a second rotationelement N2, and a first ring gear R1 internally meshed with the firstpinion P1 and being a third rotation element N3.

The second planetary gear set PG2 is a single pinion planetary gear set,and includes a second sun gear S2 being a fourth rotation element N4, asecond planet carrier PC2 rotatably supporting a second pinion P2externally meshed with the second sun gear S2 and being a fifth rotationelement N5, and a second ring gear R2 internally meshed with the secondpinion P2 and being a sixth rotation element N6.

Two rotation elements of the first planetary gear set PG1 is selectivelyconnected to the input shaft IS, three rotation elements of the secondplanetary gear set PG2 is connected to the input shaft IS and tworotation elements of the first planetary gear set PG1 through first,second, third, and fourth transfer gears TF1, TF2, TF3, and TF4, and anyone rotation element of the second planetary gear set PG2 is directlyconnected to the output shaft OS.

The first, second, third, and fourth transfer gears TF1, TF2, TF3, andTF4 respectively have first, second, third, and fourth transfer drivegears TF1 a, TF2 a, TF3 a, and TF4 a and first, second, third, andfourth transfer driven gears TF1 b, TF2 b, TF3 b, and TF4 b externallymeshed with each other.

The first transfer gear TF1 connects the second rotation element N2 withthe sixth rotation element N6.

The second transfer gear TF2 connects the third rotation element N3 withthe fifth rotation element N5.

The third transfer gear TF3 connects the input shaft IS with the fourthrotation element N4.

The fourth transfer gear TF4 connects the input shaft IS with the fourthrotation element N4.

Therefore, the rotation elements (including the first shaft IS1)connected with each other through the first, second, third, and fourthtransfer gears TF1, TF2, TF3, and TF4 are rotated in opposite directionto each other according to gear ratios of the first, second, third, andfourth transfer gears TF1, TF2, TF3, and TF4.

In addition, four clutches C1, C2, C3, and C4 and two brakes B1 and B2selectively connecting the selected rotation elements with the inputshaft IS and the transmission housing H are disposed as follows.

The first clutch C1 is disposed between the input shaft IS and the firstrotation element N1.

The second clutch C2 is disposed between the input shaft IS and thethird transfer gear TF3.

The third clutch C3 is disposed between the fourth rotation element N4and the fourth transfer gear TF4.

The fourth clutch C4 is disposed between the input shaft IS and thethird rotation element N3.

The first brake B1 is disposed between the fifth rotation element N5 andthe transmission housing H.

The second brake B2 is disposed between the fourth rotation element N4and the transmission housing H.

Both of the second clutch C2 and the third clutch C3 selectively connectthe input shaft IS with the fourth rotation element N4, but rotationspeeds transmitted to the fourth rotation element N4 through the secondclutch C2 and the third clutch C3 are different from each other sincethe gear ratio of the third transfer gear TF3 differs from the gearratio of the fourth transfer gear TF4.

The frictional elements consisting of the first, second, third, andfourth clutches C1, C2, C3, and C4 and the first and second brakes B1and B2 are conventional multi-plate friction elements of wet type thatare operated by hydraulic pressure.

FIG. 2 is an operational chart of friction elements at each shift-speedapplied to a planetary gear train according to various embodiments ofthe present invention.

As shown in FIG. 2, two frictional elements are operated at eachshift-speed in the planetary gear train according to various embodimentsof the present invention.

The first clutch C1 and the first brake B1 are operated at a firstforward speed 1ST.

The first clutch C1 and the second brake B2 are operated at a secondforward speed 2ND.

The first clutch C1 and the second clutch C2 are operated at a thirdforward speed 3RD.

The first clutch C1 and the third clutch C3 are operated at a fourthforward speed 4TH.

The first clutch C1 and the fourth clutch C4 are operated at a fifthforward speed 5TH.

The third clutch C3 and the fourth clutch C4 are operated at a sixthforward speed 6TH.

The second clutch C2 and the fourth clutch C4 are operated at a seventhforward speed 7TH.

The fourth clutch C4 and the second brake B2 are operated at an eighthforward speed 8TH.

The third clutch C3 and the first brake B1 are operated at a reversespeed REV.

FIG. 3A to FIG. 3H are lever diagrams of a planetary gear trainaccording to various embodiments of the present invention, andillustrate shift processes of the planetary gear train according tovarious embodiments of the present invention by lever analysis method.

Referring to FIG. 3A to FIG. 3H, three vertical lines of the secondplanetary gear set PG2 are set as the fourth, fifth, and sixth rotationelements N4, N5, and N6, and three vertical lines of the first planetarygear set PG1 are set as the third, the second, the first rotationelements N3, N2, and N1 from the left to the right.

A middle horizontal line represents a rotation speed of “0”, upperhorizontal lines represent positive rotation speeds and lower horizontallines represent negative rotation speeds.

“−” means that rotational elements are rotated in an opposite directionto the rotation direction of the engine. It is because the input shaftIS and the rotation elements of the first planetary gear set PG1 areexternally meshed through the first, second, third, and fourth transfergears TF1, TF2, TF3, and TF4 without an idling gear.

In addition, distances between the vertical lines of the first andsecond planetary gear sets PG1 and PG2 are set according to gear ratios(teeth number of a sun gear/teeth number of a ring gear).

Hereinafter, referring to FIG. 2 and FIG. 3A to FIG. 3H, the shiftprocesses of the planetary gear train according to various embodimentsof the present invention will be described in detail.

First Forward Speed

Referring to FIG. 2, the first clutch C1 and the first brake B1 areoperated at the first forward speed 1ST.

As shown in FIG. 3A, a rotation speed of the input shaft IS is input tothe first rotation element N1 by operation of the first clutch C1 andthe third rotation element N3 is operated as a fixed element byoperation of the first brake B1. Therefore, the rotation elements of thefirst planetary gear set PG1 form a first speed line T1.

In addition, the fifth rotation element N5 is operated as a fixedelement by operation of the first brake B1 and a rotation speed of thesecond rotation element N2 is changed according to the gear ratio of thefirst transfer gear TF1 and is then input to the sixth rotation elementN6 as inverse rotation speed. Therefore, the rotation elements of thesecond planetary gear set PG2 form a first shift line SP1 and D1 isoutput through the sixth rotation element N6 that is the output element.

Second Forward Speed

The first brake B1 that was operated at the first forward speed 1ST isreleased and the second brake B2 is operated at the second forward speed2ND.

As shown in FIG. 3A, the rotation speed of the input shaft IS is inputto the first rotation element N1 by operation of the first clutch C1 andthe fourth rotation element N4 is operated as a fixed element byoperation of the second brake B2.

Therefore, the rotation elements of the first planetary gear set PG1form a second speed line T2 and the rotation elements of the secondplanetary gear set PG2 form a second shift line SP2 by cooperation ofthe first planetary gear set PG1 and the second planetary gear set PG2.Therefore, D2 is output through the sixth rotation element N6 that isthe output element.

Third Forward Speed

The second brake B2 that was operated at the second forward speed 2ND isreleased and the second clutch C2 is operated at the third forward speed3RD.

As shown in FIG. 3B, the rotation speed of the input shaft IS is inputto the first rotation element N1 by operation of the first clutch C1. Inaddition, the rotation speed of the input shaft IS is convertedaccording to the gear ratio of the third transfer gear TF3 and is theninput to the fourth rotation element N4 by operation of the secondclutch C2.

Therefore, the rotation elements of the first planetary gear set PG1form a third speed line T3 and the rotation elements of the secondplanetary gear set PG2 form a third shift line SP3 by cooperation of thefirst planetary gear set PG1 and the second planetary gear set PG2.Therefore, D3 is output through the sixth rotation element N6 that isthe output element.

Fourth Forward Speed

The second clutch C2 that was operated at the third forward speed 3RD isreleased and the third clutch C3 is operated at the fourth forward speed4TH.

As shown in FIG. 3C, the rotation speed of the input shaft IS is inputto the first rotation element N1 by operation of the first clutch C1. Inaddition, the rotation speed of the input shaft IS is convertedaccording to the gear ratio of the fourth transfer gear TF4 and is theninput to the fourth rotation element N4 by operation of the third clutchC3.

Therefore, the rotation elements of the first planetary gear set PG1form a fourth speed line T4 and the rotation elements of the secondplanetary gear set PG2 form a fourth shift line SP4 by cooperation ofthe first planetary gear set PG1 and the second planetary gear set PG2.Therefore, D4 is output through the sixth rotation element N6 that isthe output element.

Fifth Forward Speed

The third clutch C3 that was operated at the fourth forward speed 4TH isreleased and the fourth clutch C4 is operated at the fifth forward speed5TH.

As shown in FIG. 3D, the rotation speed of the input shaft IS is inputto the first rotation element N1 and the third rotation element N3 byoperation of the first clutch C1 and the fourth clutch C4. Therefore,the first planetary gear set PG1 becomes a direct-coupling state.

In addition, the rotation speed of the third rotation element N3 isconverted according to the gear ratio of the second transfer gear TF2and is then input to the fifth rotation element N5 as an inverserotation speed.

Therefore, the rotation elements of the first planetary gear set PG1form a fifth speed line T5 and the rotation elements of the secondplanetary gear set PG2 form a fifth shift line SP5 by cooperation of thefirst planetary gear set PG1 and the second planetary gear set PG2.Therefore, D5 is output through the sixth rotation element N6 that isthe output element.

Sixth Forward Speed

The first clutch C1 that was operated at the fifth forward speed 5TH isreleased and the third clutch C3 is operated at the sixth forward speed6TH.

As shown in FIG. 3E, the rotation element of the input shaft IS is inputto the third rotation element N3 by operation of the fourth clutch C4.In addition, the rotation element of the input shaft IS is convertedaccording to the gear ratio of the fourth transfer gear TF4 and is theninput to the fourth rotation element N4 by operation of the third clutchC3.

Therefore, the rotation elements of the first planetary gear set PG1form a sixth speed line T6 and the rotation elements of the secondplanetary gear set PG2 form a sixth shift line SP6 by cooperation of thefirst planetary gear set PG1 and the second planetary gear set PG2.Therefore, D6 is output through the sixth rotation element N6 that isthe output element.

Seventh Forward Speed

The third clutch C3 that was operated at the sixth forward speed 6TH isreleased and the second clutch C2 is operated at the seventh forwardspeed 7TH.

As shown in FIG. 3F, the rotation speed of the input shaft IS is inputto the third rotation element N3 by operation of the fourth clutch C4.In addition, the rotation speed of the input shaft IS is convertedaccording to the gear ratio of the third transfer gear TF3 and is theninput to the fourth rotation element N4 by operation of the secondclutch C2.

Therefore, the rotation elements of the first planetary gear set PG1form a seventh speed line T7 and the rotation elements of the secondplanetary gear set PG2 form a seventh shift line SP7 by cooperation ofthe first planetary gear set PG1 and the second planetary gear set PG2.Therefore, D7 is output through the sixth rotation element N6 that isthe output element.

Eighth Forward Speed

The second clutch C2 that was operated at the seventh forward speed 7THis released and the second brake B2 is operated at the eighth forwardspeed 8TH.

As shown in FIG. 3G, the rotation speed of the input shaft IS is inputto the third rotation element N3 by operation of the fourth clutch C4and the fourth rotation element N4 is operated as the fixed element byoperation of the second brake B2.

Therefore, the rotation elements of the first planetary gear set PG1form an eighth speed line T8 and the rotation elements of the secondplanetary gear set PG2 form an eighth shift line SP8 by cooperation ofthe first planetary gear set PG1 and the second planetary gear set PG2.Therefore, D8 is output through the sixth rotation element N6 that isthe output element.

Reverse Speed

As shown in FIG. 2, the third clutch C3 and the first brake B1 areoperated at the reverse speed REV.

As shown in FIG. 3H, the rotation speed of the input shaft IS isconverted according to the gear ratio of the fourth transfer gear TF4and is then input to the fourth rotation element N4 by operation of thethird clutch C3, and the fifth rotation element N5 is operated as thefixed element by operation of the first brake B1.

Therefore, the rotation elements of the first planetary gear set PG1form a reverse speed line Tr and the rotation elements of the secondplanetary gear set PG2 form a reverse shift line RS. Therefore, REV isoutput through the sixth rotation element N6 that is the output element.

As described above, the planetary gear train according to variousembodiments of the present invention can achieve eight forward speedsand one reverse speed by combining two planetary gear sets PG1 and PG2being the simple planetary gear sets, four transfer gears TF1, TF2, TF3,and TF4 being the externally-meshed gears and six frictional elementsC1, C2, C3, C4, B1, and B2.

In addition, optimum gear ratios may be set due to ease of changing gearratios by using four transfer gears being externally-meshed gears aswell as the planetary gear sets. Since gear ratios can be changedaccording to target performance, starting performance, power deliveryperformance and fuel economy may be improved. Therefore, a start-upclutch instead of a torque converter may be used.

In addition, two friction elements are operated at each shift-speed andone friction element is released and another friction element isoperated so as to shift to a neighboring shift-speed. Therefore, shiftcontrol condition is fully satisfied.

FIG. 4 is a schematic diagram of a planetary gear train according tovarious embodiments of the present invention.

Referring to FIG. 4, the second clutch C2 is disposed between the fourthrotation element N4 and the third transfer gear TF3, in contrast to FIG.1 which shows the second clutch C2 is disposed between the input shaftIS and the third transfer gear TF3.

Since functions of this illustrated embodiment are similar to those ofthe above-described embodiments except the position of the second clutchC2, detailed description thereof will be omitted.

FIG. 5 is a schematic diagram of a planetary gear train according tovarious embodiments of the present invention.

Referring to FIG. 5, the first brake B1 is disposed between the thirdrotation element N3 and the transmission housing H, in contrast to FIG.1, which shows the first brake B1 is disposed between the fifth rotationelement N5 and the transmission housing H.

Since functions of this illustrated embodiment are similar to those ofthe above-described embodiments except the position of the first brakeB1, detailed description thereof will be omitted.

FIG. 6 is a schematic diagram of a planetary gear train according tovarious embodiments of the present invention.

Referring to FIG. 6, the first planetary gear set PG1 is a double pinionplanetary gear set, in contrast to FIG. 1, which shows the firstplanetary gear set PG1 is the single pinion planetary gear set.

Therefore, the second rotation element N2 is changed from the firstplanet carrier PC1 to the first ring gear R1 and the third rotationelement N3 is changed from the first ring gear R1 to the first planetcarrier PC1.

Since functions of this illustrated embodiment are similar to those ofthe above-described embodiments except the rotation elements consistingof the second and third rotation elements N2 and N3, detaileddescription thereof will be omitted.

Eight forward speeds and one reverse speed can be achieved by combiningtwo planetary gear sets being the simple planetary gear sets, fourtransfer gears and six frictional elements.

In addition, since two planetary gear sets are disposed separately onthe input shaft and the output shaft disposed in parallel with apredetermined distance, a length thereof may be reduced and mountabilitymay be improved.

In addition, optimum gear ratios may be set due to ease of changing gearratios by using four external-meshing gears as well as the planetarygear sets. Since gear ratios can be changed according to targetperformance, starting performance, power delivery performance, and fueleconomy may be improved. Therefore, a start-up clutch instead of atorque converter may be used.

In addition, two friction elements are operated at each shift-speed, andone friction element is released and another friction element isoperated so as to shift to a neighboring shift-speed. Therefore, shiftcontrol condition is fully satisfied.

For convenience in explanation and accurate definition in the appendedclaims, the terms upper or lower, and etc. are used to describe featuresof the exemplary embodiments with reference to the positions of suchfeatures as displayed in the figures.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described in orderto explain certain principles of the invention and their practicalapplication, to thereby enable others skilled in the art to make andutilize various exemplary embodiments of the present invention, as wellas various alternatives and modifications thereof. It is intended thatthe scope of the invention be defined by the Claims appended hereto andtheir equivalents.

1. A planetary gear train of an automatic transmission for a vehicle,comprising: an input shaft receiving engine torque; an output shaftparallel with the input shaft at a predetermined distance and outputtingchanged torque; a first planetary gear set disposed on the input shaft,and including a first rotation element selectively connected to theinput shaft, a second rotation element operated as an output element,and a third rotation element selectively connected to the input shaft; asecond planetary gear set disposed on the output shaft, and including afourth rotation element selectively connected to the input shaft throughtwo paths having different gear ratios and selectively connected to atransmission housing, a fifth rotation element selectively connected tothe input shaft and the transmission housing, and a sixth rotationelement connected to the second rotation element and directly connectedto the output shaft; four transfer gears externally meshing the fourth,fifth, and sixth rotation elements with the input shaft, the secondrotation element or the third rotation element; and frictional elementsincluding clutches selectively connecting the first, third, and fourthrotation elements with the input shaft and brakes selectively connectingthe fourth and fifth rotation elements with the transmission housing. 2.(canceled)
 3. The planetary gear train of claim 1, wherein: the firstplanetary gear set is a single pinion planetary gear set having a firstsun gear being the first rotation element, a first planet carrier beingthe second rotation element, and a first ring gear being the thirdrotation element; the second planetary gear set is a single pinionplanetary gear set having a second sun gear being the fourth rotationelement, a second planet carrier being the fifth rotation element, and asecond ring gear being the sixth rotation element; and the four transfergears comprise: a first transfer gear including a first transfer drivegear connected to the second rotation element and a first transferdriven gear connected to the sixth rotation element; a second transfergear including a second transfer drive gear connected to the thirdrotation element and a second transfer driven gear connected to thefifth rotation element; a third transfer gear including a third transferdrive gear connected to the input shaft and a third transfer driven gearconnected to the fourth rotation element; and a fourth transfer gearincluding a fourth transfer drive gear connected to the input shaft anda fourth transfer driven gear connected to the fourth rotation element.4-5. (canceled)
 6. The planetary gear train of claim 3, wherein thefrictional elements comprise: a first clutch disposed between the inputshaft and the first rotation element; a second clutch disposed betweenthe third transfer gear and the fourth rotation element; a third clutchdisposed between the fourth transfer gear and the fourth rotationelement; a fourth clutch disposed between the input shaft and the thirdrotation element; a first brake disposed between the fifth rotationelement and the transmission housing; and a second brake disposedbetween the fourth rotation element and the transmission housing.
 7. Theplanetary gear train of claim 3, wherein the frictional elementscomprise: a first clutch disposed between the input shaft and the firstrotation element; a second clutch disposed between the input shaft andthe third transfer gear; a third clutch disposed between the fourthtransfer gear and the fourth rotation element; a fourth clutch disposedbetween the input shaft and the third rotation element; a first brakedisposed between the third rotation element and the transmissionhousing; and a second brake disposed between the fourth rotation elementand the transmission housing.
 8. The planetary gear train of claim 1,wherein the first planetary gear set is a double pinion planetary gearset including a first sun gear being the first rotation element, a firstring gear being the second rotation element, and a first planet carrierbeing the third rotation element; and the second planetary gear set is asingle pinion planetary gear set including a second sun gear being thefourth rotation element, a second planet carrier being the fifthrotation element, and a second ring gear being the sixth rotationelement.
 9. A planetary gear train of an automatic transmission for avehicle, comprising: an input shaft receiving engine torque; an outputshaft parallel with the input shaft with a predetermined distance andoutputting changed torque; a first planetary gear set disposed on theinput shaft, and including a first rotation element selectivelyconnected to the input shaft, a second rotation element, and a thirdrotation element selectively connected to the input shaft; a secondplanetary gear set disposed on the output shaft, and including a fourthrotation element selectively connected to the input shaft through twopaths having different gear ratios and selectively connected to atransmission housing, a fifth rotation element selectively connected tothe input shaft and the transmission housing, and a sixth rotationelement connected to the second rotation element and directly connectedto the output shaft; a first transfer gear connecting the secondrotation element with the sixth rotation element; a second transfer gearconnecting the third rotation element with the fifth rotation element; athird transfer gear connecting the input shaft with the fourth rotationelement; a fourth transfer gear connecting the input shaft with thefourth rotation element; and frictional elements including clutchesselectively connecting the first, third, and fourth rotation elementswith the input shaft and brakes selectively connecting the fourth andfifth rotation elements with the transmission housing.
 10. (canceled)11. The planetary gear train of claim 9, wherein the first planetarygear set is a double pinion planetary gear set including a first sungear being the first rotation element, a first ring gear being thesecond rotation element, and a first planet carrier being the thirdrotation element; and the second planetary gear set is a single pinionplanetary gear set including a second sun gear being the fourth rotationelement, a second planet carrier being the fifth rotation element, and asecond ring gear being the sixth rotation element.
 12. (canceled) 13.The planetary gear train of claim 9, wherein the frictional elementscomprise: a first clutch disposed between the input shaft and the firstrotation element; a second clutch disposed between the third transfergear and the fourth rotation element; a third clutch disposed betweenthe fourth transfer gear and the fourth rotation element; a fourthclutch disposed between the input shaft and the third rotation element;a first brake disposed between the fifth rotation element and thetransmission housing; and a second brake disposed between the fourthrotation element and the transmission housing.
 14. The planetary geartrain of claim 9, wherein the frictional elements comprise: a firstclutch disposed between the input shaft and the first rotation element;a second clutch disposed between the input shaft and the third transfergear; a third clutch disposed between the fourth transfer gear and thefourth rotation element; a fourth clutch disposed between the inputshaft and the third rotation element; a first brake disposed between thethird rotation element and the transmission housing; and a second brakedisposed between the fourth rotation element and the transmissionhousing.